1. Field of the Invention
The present invention relates to an audio device, and more particularly to a circuit for monitoring the recording state of audio signals in a magnetic tape.
2. Description of the Prior Art
As a widely used method for recording audio signals on a certain medium and for playing back the recorded signals, there is a method in which electric audio signals are stored in a magnetic tape as magnetic signals by using an audio head, and if necessary, the magnetic signals are converted into electric audio signals by the audio head again.
Referring to FIG. 1, there is shown a circuit diagram of a conventional audio device in an audio head and a magnetic tape are utilized for recording audio signals on the magnetic tape and playing back the recorded audio signals. The audio device comprises an audio signal record and detect circuit 100, a first amplifying circuit 200, a second amplifying circuit 300, and a third amplifying circuit 400.
The audio signal record and detect circuit 100 which is adapted to selectively record audio signals on a magnetic tape or detect them. That is, the audio signal record and detect circuit 100 includes a mixer 10 to which the output from the third amplifying circuit 400 is applied. The mixer 10 comprises an oscillator 11 for oscillating a bias signal and an adder 12 for mixing the output from the third amplifying circuit 400 and the output from the oscillator 11. The audio signal record and detect circuit 100 also includes a switch SW2 to which the mixer 10 is connected so that its output is selectively applied to an audio head AHD, according to the switching operation of the switch SW2. The circuit 100 also comprises an audio head AHD for detecting signals recorded on the magnetic tape.
The first amplifying circuit 200 comprises a switch SW1 for selectively connecting the output of the audio signal record and detect circuit 100 and the first amplifying circuit 200 and an OP amplifier OP1 for amplifying the output sent from the circuit 100 via the switch SW1.
The third amplifying circuit 400 comprises a switch SW3 connected at its fixed terminals P and R with the output of the first amplifying circuit 200 and the output of the second amplifying circuit 300, respectively, and OP amplifiers OP2, OP3 and OP4. In particular, the OP amplifier OP4 is connected at its input and output to the OP amplifier OP2 and the mixer 10, respectively.
All of switches SW1 to SW3 are operatively connected with one another, as shown in FIG. 1.
This device having the above-mentioned arrangement is operated in a record mode for recording audio signals received from a microphone MIC on a magnetic tape or in a playback mode for playing back the audio signals recorded on the magnetic tape, for their listening. Now, the operation of the device will be described in conjunction with respective modes.
1) RECORD MODE
When the audio device should be operated in its record mode, the user operates switches SW1, SW2 and SW3 operatively connected with one another so that their movable terminals are connected to the corresponding fixed terminals R, respectively. Accordingly, an audio signal from the microphone MIC is sent to the second amplifying circuit 300 and amplified by the second amplifying circuit 300. The amplified audio signal is then applied to the third amplifying circuit 400, via the switch SW3. The audio signal applied to the third amplifying circuit 400 is amplified in the OP amplifier OP2 which is a linear amplifier and then applied to OP amplifiers OP3 and OP4.
The OP amplifier OP3 is a monitor amplifier which functions to amplify the audio signal to the level suitable for driving an outer earphone EP or a speaker SPK. On the other hand, the OP amplifier OP4 amplifies the audio signal to the level suitable for recording it on the magnetic tape by an audio head and applies the amplified audio signal to the mixer 10. The adder 12 of mixer 10 mixes the output from the oscillator 11 generating a bias signal with a certain frequency and the output from the OP amplifier OP4. As a result, the audio signal from the microphone can be stored in the magnetic tape by the audio head AHD and via the switch SW2 of the audio signal record and detect circuit 100.
Since the audio signal from the microphone MIC is also sent to the outer earphone, via the OP amplifier OP3, the user can check whether the audio signal received from the microphone MIC is good, by listening to the sound from the earphone.
2) PLAYBACK MODE
When the audio device should be operated in its playback mode, the user operates switches SW1, SW2 and SW3 so that their movable terminals are connected to the corresponding fixed terminals P, respectively. Accordingly, an audio signal by the audio head AHD is sent to the first amplifying circuit 200 via the switch SW1 and amplified by the OP amplifier OP1 of the first amplifying circuit 200. The amplified audio signal is then applied to the third amplifying circuit 400, via the switch SW3. Thereby, the user can listen to the sound played back from the audio signal recorded on the magnetic tape.
During the operation of the audio device in the playback mode, an audio signal possibly sent from the microphone MIC is prevented from being applied to the third amplifying circuit 400, by virtue of the switch SW3. The switch SW2 functions to prevent the output from the mixer 10 to be applied to the audio head AHD.
According to the arrangement of the conventional audio device, the user can monitor whether an audio signal is outputted from the microphone, in that in record mode operation, the third amplifying circuit amplifies the audio signal from the microphone and sends the amplified audio signal to the earphone. However, it is impossible to confirm whether the received audio signal is correctly recorded on the magnetic tape by the audio head. If an abnormality occurs at the mixer or the audio head, the audio signal can not correctly recorded on the magnetic tape. In spite of this, the user continuously attempts to operate the audio device in its record mode for recording the audio signal on the magnetic tape, since he can not recognize the abnormality by monitoring only the audio sound from the earphone. As a result, he can recognize the record of no audio signal, only under the playback mode operation of the audio device.